When someone asks you, "What time is it?," the answer's easy. Just look at your watch! But "What is time?" is a little trickier. We think of time as a moment or period that can be measured by clocks and calendars. In his General Theory of Relativity, Einstein explained that time is the fourth dimension in what's called "space-time."

Image credits: main image, courtesy of Armistead Page Booker Jr.; courtesy of AMNH Department of Library Services 281479.